1. Field of the Invention
The major application for the invention is roller pumps used for extracorporeal circuits to pump physiological fluids and peristaltic pumps for industry. The description of the invention hereinafter makes extensive use of cardiopulmonary bypass applications. This emphasis is made for description only. It should be understood that the described art is easily extended to other applications utilizing roller pumps. The invention discloses the combination of shunt, with or without a pressure loaded valve, connecting the inlet and outlet of the tubing used in peristaltic pumps, said shunt used to limit pump inlet pressure to safe levels. The resistance of the shunt (and/or valve) to flow is such that the shunt flow is low compared to pump flow when the pump tube is full but increases to a significant amount as the pump tube collapses and/or as the pressure difference between pump inlet and pump outlet increases.
2. Description of the Prior Art.
The simplicity and availability of the standard roller pump have made it the choice for dialysis and routine cardiopulmonary bypass. The standard tubing used to pump blood with the roller pump is almost always polyvinyl chloride (PVC) with a nominal size from 1/4" internal diameter (ID) with a 1/16" wall, to 1/2" ID with a 3/32" wall. Although soft tubing (e.g. 40 to 55 Shore A) are available, tubing with a more rigid wall (e.g., 65 to 75 Shore A) are used almost exclusively. Use of thick, more rigid walled tubing results in the roller pump maintaining a constant flow independent of clinically expected changes in inlet pressure. Thus, a decrease in blood supply at the pump inlet, without a concomitant decrease in pump speed, can cause excessive suction, leading to air embolism, thrombosis and damage by the "venous" cannula to the patient's intima. Tests have shown that when starting at equilibrium using a PVC tube with 1/4" ID and 1/16" wall, a 30% drop in inlet flow caused the inlet pressure to drop from 0 to -450 mmHg.
When the roller pump is used during cardiac surgery for venting the heart (e.g., left atrium, left or right ventricle) or for collecting shed blood from the chest cavity it requires the constant surveillance by a trained perfusionist to assure that excess suction does not occur. Alternatively, a suction relief valve (e.g. RLV-2100 "B" American Omni Medical Inc., Costa Mesa Calif. 92626, U.S. Pat. No. 4,758,224 and William Harvey's H-130 Overpressure Safety Valve, C. R. Bard Inc., Tewksbury, Mass. U.S. Pat. No. 4,502,502) can be placed at the pump inlet. These valves relieve suction at the pump inlet by incorporating a preloaded one way valve that introduces air into the blood. However, air introduction into the blood stream result in an annoying whistling sound and may increase hemolysis. These valves also prevent blood from being pumped into the heart by an accidental reversal of the pump rotational direction with a one way duck bill valve (e.g. Model VL614-101, Vernay Lab. Yellow Spring Ohio). However, if such eversal occurs, blood spatters out of the valve as far as 10ft, a major infectious hazard o operating room personnel.
U.S. Pat. Nos. 4,515,589, 4,767,289 and 4,650,471 describe devices to be used with the roller pump to regulate flow as a function of inlet pressure that utilize thin wall tubing (e.g. 0.375" ID with a 0.010" wall with ID/Wall=37.5) housed inside standard thick wall tubing. One (U.S. Pat. N. 4,767,289) also incorporates a one way valve at the pump outlet to prevent accidental back flow to the patient. These devices are expensive to manufacture and therefore have not been commercially successful.